This invention relates to a mechanism for longitudinally sealing an elongated bag-making material without leaving unsightly wrinkles thereon.
A so-called vertical pillow type packaging machine is a kind of bag maker-packaging machine adapted to bend a web of thermoplastic bag-making material (herein referred to as "the film") into a cylindrical shape, to use a heater for longitudinally sealing together the mutually overlapping side edge parts of the film while the cylindrically formed film is being pulled along the outer surface of a so-called filling cylinder through which articles to be packaged are dropped from above, and then to use another heater to seal the cylindrically formed film transversely to the direction of its motion, as disclosed, for example, in Japanese Patent Publication Tokkai 5-77805, such that the filling of the materials and making of packages can be carried out in one continuous series of operations.
The thermoplastic bag-making material used in this type of package production is usually of a layered structure. In the middle is a vapor deposited aluminum layer, or a printed layer, having on the side of its front surface a polypropylene layer of the so-called OPP type which has lustre but is poor in expandability and on the side of its back surface a so-called CPP type polypropylene layer which seals well but tends to shrink when heated. When heat is applied on such a multi-layered film, wrinkles with V-shaped bars, sometimes called "the chevrons", are formed as indicated by letter c in FIG. 2 on both sides of the seal area indicated by letter a because the front surface and the back surface of the film F have different expansion characteristics. As a result, the printed surface becomes deformed, affecting adversely the merchantability of the product.